# Symmetrization of Strong Hydrogen Bond under High Pressure in Bihydroxide-Ion-Containing NaCu2(SO4)2·H3O2 Revealed by Experimental Charge Density, Single-Crystal Electron Diffraction, and Neutron Diffraction Studies

## TL;DR

This study explores how hydrogen bonds in a mineral compound become symmetric under high pressure, using advanced techniques to track changes in electron density and atomic behavior.

## Contribution

The paper reveals hydrogen bond symmetrization in natrochalcite at the lowest pressure reported in inorganic compounds, using novel experimental methods.

## Key findings

- Natrochalcite undergoes hydrogen bond symmetrization at unprecedented low pressure.
- Electron density redistribution during the process is traced via multipole refinement and Laplacian analysis.
- Atomic equations of state are derived from volume-pressure relationships of individual atoms.

## Abstract

In minerals and inorganic compounds, strong hydrogen
bonding can
lead to the formation of complex ionic species such as the H3O2
– bihydroxide anion and Zundel cation
H5O2
+. We studied [NaCu2(SO4)2·H3O2] natrochalcite,
which contains bihydroxide anions and undergoes hydrogen bond symmetrization
at the lowest pressure reported so far among inorganic compounds.
Hydrogen bond symmetrization leads to changes in the bulk modulus,
seismic wave velocities, and proton mobility and plays a primary role
in high-temperature superconductivity, but its characteristics are
not well understood due to a lack of systematic studies and limitations
of experimental methods sensitive to this subtle change. In this work,
we applied experimental charge density analysis based on in situ single-crystal
X-ray diffraction data, along with the single-crystal neutron and
electron diffraction experiments, to probe the behavior of hydrogen
atoms during the hydrogen bond symmetrization process under high-pressure
conditions. On the way to the symmetrical H-bonding, natrochalcite
undergoes a series of complex redistributions of electron density,
which we trace with multipole refinement and detailed analysis of
changes in the Laplacian of electron density values. Additionally,
we deconvoluted the equation of state (volume of the unit cell vs
pressure relation) into the atomic equation of states describing dependencies
of atomic charges or volumes vs pressure.